Nationals president and general manager Mike Rizzo announced the reigning World Series champions' decision and called on Major League Baseball to "work quickly to resolve issues with their process and their lab."
Astros GM James Click also emphasized his team's priority of player health and safety, but he said the defending American League champions are "optimistic that this process will be ironed out and we'll be back on the field and ready to compete for a championship soon."
Cardinals president of baseball operations John Mozeliak told reporters that there's been "a little frustration" about the lack of results.
Also, test administrators did not arrive to the Los Angeles Angels' facilities in Anaheim, California, or Long Beach on Sunday, prompting the team to collect its own saliva samples and mail them to MLB's testing facility in Utah, multiple sources told ESPN's Alden Gonzalez.
The Angels pushed their workout back by four hours on Monday, from 9 a.m. PT to 1 p.m., and made it optional. Angels manager Joe Maddon described it as the equivalent of a rain delay and believes the hiccup in testing was "a short-lived situation."
"I think it has a lot to do with the Fourth of July," Maddon said. "That's my opinion. I believe things are gonna get straightened out relatively soon. I have not lost any confidence."
MLB addressed the delays in a statement Monday, saying that the results for 98% of the samples taken from June 27 to July 3 have been reported as of Sunday night.
Eighty-six samples out of 3,740 remain pending as of Monday morning.
"Our plan required extensive delivery and shipping services, including proactive special accommodations to account for the holiday weekend," the statement read. "The vast majority of those deliveries occurred without incident and allowed the protocols to function as planned.
"Unfortunately, several situations included unforeseen delays. We have addressed the delays caused by the holiday weekend and do not expect a recurrence."
The Arizona Diamondbacks pushed back Monday's workout a few hours, hoping to get additional results from tests taken on Friday.
"I think if we really want this to succeed, we're going to have to figure that out," said Chicago Cubs third baseman Kris Bryant, who voiced concern that some teammates haven't been tested in a week. "If you can't really nail the easy part, which is right now ... we've got a big hill to climb."
Meanwhile, players continue to test positive or opt out of playing.
Texas Rangers All-Star outfielder Joey Gallo was isolating at his Dallas apartment Monday after two positive test results sandwiched around a negative. General manager Jon Daniels said Gallo was asymptomatic.
"You had to have the mindset that there were going to be some kinks to be worked out," Daniels said. "It's just not realistic to roll out an operation of this scale on as quick a timeline as we've had and not deal with some challenges."
Diamondbacks outfielder Kole Calhoun also tested positive but is asymptomatic and feels good, manager Torey Lovullo said.
Atlanta Braves outfielder Nick Markakis became the latest high-profile player to choose not to play this season. Los Angeles Dodgers pitcher David Price, Colorado Rockies infielder Ian Desmond, Nationals first baseman Ryan Zimmerman and Braves pitcher Felix Hernandez are among those who have opted out.
Multiple Blue Jays players didn't make the trip to Toronto for training camp after one player tested positive for the coronavirus, according to an official familiar with the matter, who spoke on condition of anonymity because they were not authorized to speak publicly about it. Players who were in contact with the unnamed player will remain in Florida for further testing and take a charter later in the week if cleared.
Rizzo said that all Nationals players and staff were tested Friday but that they have not received the results. He also said that without timely test results, it is "not safe for us to continue with Summer Camp" and suggested that the 2020 season is "at risk."
"Per MLB's protocol, all players and staff were tested for Covid-19 on Friday, July 3rd. Seventy-two hours later, we have yet to receive the results of those tests," Rizzo said in a statement Monday morning. "We cannot have our players and staff work at risk. Therefore, we have cancelled our team workout scheduled for this morning. We will not sacrifice the health and safety of our players, staff and their families. Without accurate and timely testing it is simply not safe for us to continue with Summer Camp. Major League Baseball needs to work quickly to resolve issues with their process and their lab. Otherwise, Summer Camp and the 2020 Season are at risk."
The Astros announced hours later that they had received the delayed test results and would work out Tuesday.
The Nationals announced Sunday that two players out of 60 tested had turned up positive for the coronavirus. But according to manager Dave Martinez, those two players took their tests last Wednesday, before reporting to Nationals Park for summer camp.
Washington reliever Sean Doolittle also spoke out Sunday, lamenting not having his COVID-19 test results back and imploring MLB to "clean this up."
Doolittle said he is still debating whether to play this season, weighing safety concerns and physical and mental health.
Milwaukee Brewers manager Craig Counsell said things are going smoothly with his club.
"There's a lot of hard work that goes into the logistics of setting this up," Counsell said. "There's going to be snags in this. I think we all know that. But I'm pleased with how it's gone so far."
Miami Marlins manager Don Mattingly said players and staff were tested again Monday because of testing issues and they'd consider canceling workouts if the problem persisted.
"I can't say I'm that confident because I haven't been shown yet. It's kind of tough to be confident in something that hasn't proved to be foolproof," Betts said. "There's not a whole lot really I can do; it's out of my control. But it's in someone's control, and whoever's control it's in has to find a way to make it work or this whole operation may not be able to work."
The Cleveland Indians have not had any issues related to testing, but All-Star shortstop Francisco Lindor said it's vital that teams get equal practice time before the season starts.
"We've got to make sure the testing is 99.9% accurate," Lindor said. "We have to do a better job as players, the union, drug testing people, everybody. If we're going to make this work, we have to do a better job. There's no way you can suspend practices. That's not a fair advantage for anybody. We all signed up to have a 21-day spring training, that's how we should do it. Everybody should have the same amount of days."
The Associated Press contributed to this report.
MLB's early COVID-19 testing not running smoothly
Jeff Passan explains how MLB is struggling to implement coronavirus tests thus far.